The prior art commonly employs broadband integrated circuit IF amplifiers in radio communication receivers, with discrete frequency-selective filtering between the IF amplifiers. Typically, an integrated circuit detector immediately follows the final IF amplifier stage on a common silicon chip. Thus, there is virtually no practical method of filtering the final amplified IF signal just prior to its detection. In such cases, the noise reaching the detector from the final IF amplifier is relatively broadband noise, and the S/N ratio of the overall system is thus degraded by an amount proportional to the ratio of final stage amplifier bandwidth to the channel (or system) IF bandwidth. Especially where small signal levels are involved, this degradation in S/N may be critical.
One prior art solution is to place a large amount of the required overall gain ahead of the final integrated circuit amplifier (where it can still be passed through a relatively narrow band discrete frequency filter). This improves the signal-to-noise ratio by increasing the signal, not by reducing the noise. Readily apparent drawbacks to this solution include increased power requirements and increased cost. Alternatively, an extra noise band limiting filter could be employed prior to the detector, but this also increases cost and requires at least two extra package pins, thus increasing the cost and complexity of the integrated circuit.
Another (more sophisticated) prior approach to the problem involves improving the overall system's noise figure by controlling channel bandwidth. Examples of such prior art are:
U.S. Pat. No. 3,909,729--Baghdady (Sept. 1975)
U.S. Pat. No. 3,217,257--Boatwright (Nov. 1965)
U.S. Pat. No. 2,969,459--Hern (Jan. 1961)
In general, such prior art has relied on active feedback control loops to achieve variable channel bandwidth control for selected circuit areas. For example, the three above-cited patents all utilize this technique.
Any reduction in the final IF amplifier bandwidth will result in an improved S/N figure and thus reduce the amount of IF gain required ahead of the final IF amplifier. This is a desirable result (insofar as S/N alone is concerned) because it allows the IF gain and filtering to be concentrated, reduces the number of circuits required, and thus the size and cost of the receiver.
It is therefore highly desirable to reduce the final IF stage of integrated circuit amplifier bandwidth at low signal levels where noise considerations are most important. Since inductors and very large capacitors are very difficult to realize in integrated circuit form, simple resistance/capacitance (R/C) time constants are preferred to tuned RLC (resistance/inductive/capacitance) circuits.
At higher signal levels noise is less important but distortion caused by amplitude-to-phase conversion can be a problem. This problem will be compounded by any added capacitance (e.g. of the bandwidth reduction circuitry), since varying drive to the transistors in any stage will then cause greater variation in the detector output waveform due to reduced slew-rate capability.